Nonsustaining hybrid propellant grain

ABSTRACT

A nonsustaining hybrid propellant grain is provided for use in hybrid rocket systems employing halogen and interhalogen oxidizers. The propellant grain comprises an organic binder and a nitrate oxidizing salt in amounts substantially stoichiometric with the carbon content of the grain. The preferred nitrate is ammonium nitrate.

United States Patent 1 1 3,698,191 Ebeling, Jr. 1451 Oct. 17, 1972 [54]NONSUSTAINING HYBRID 3,068,641 12/1962 Fox ..60/220 PROPELLANT GRAIN3,083,526 2/1963 Hudson ..60/220 3,147,161 9/1964 Abele etal ..149/19[721 g Kutzmwm 3,234,729 2/1966 Altman et a1 ..60/220 3,257,802 6/1966Kaufman ..60/220 [73] Assignee: United Aircraft Corporation, Ea t3,305,523 2/1967 Burnside ..149/19X Hartford, Conn. l Filed: g 1967Primary Exammer-Ben am1n R. Padgett Appl. No.: 660,869

us. 131 ..60/220, 60/219 Int. c1. ..C06d 5/10 Field 61 Search ..60/218,219; 149/2, 19

Attorney-Steven F. Stone [57] ABSTRACT A nonsustaining hybrid propellantgrain is provided for use in hybrid rocket systems employing halogen andinterhalogen oxidizers. The propellant grain comprises an organic binderand a nitrate oxidizing salt in amounts substantially stoichiometricwith the carbon content of the grain. The preferred nitrate is ammoniumnitrate.

6 Claims, N0 Drawings NONSUSTAINING HYBRID PROPELLANT GRAIN BACKGROUNDOF THE INVENTION Hybrid propellant systems generally consist of acombustion chamber containing a solid fuel grain and an injectorassembly for injecting a fluid oxidizer into the combustion chamber toreact with the fuel grain and produce combustion gases which are thenexhausted through a nozzle to produce thrust. Hybrid propellant systemsare simpler than liquid bipropellant systems in that tankage and flowcontrol systems are required for only one propellant and have a decidedadvantage over solid propellant systems in that throttling and on-offcapability can readily be obtained by control of the flow of thefluidoxidizer. In order to increasethe performance of such systems, theprior art has progressed from the relatively low energy oxygen typeoxidizers such as nitric acid and oxygen to high energy halogen typeoxidizers such as fluorine and the interhalogen type compounds of whichClF BrF and BrF are exemplary.

With respect to the fuel component, the simplest fuel system consists ofa cured polymeric grain made from a suitable polymeric system such aspolyethylene, polypropylene, polybutadiene, polyurethane, polyester, andpolyisobutylene, for example. In some cases, an amount of an inorganicoxidizing agent is added to increase the regression rate of the grain.It is known, however, that higher performance can be obtained if, inaddition to the polymeric binder, various oxidizable materials such asmetals; e.g., aluminum and magnesium; metal hydrides; boron; carboranecompounds; and high energy organic compounds such as organic substitutedhydrazine and boron hydride compounds, and various heterocyclic nitrogencontaining compounds such as, for example, tetraformaltrisazine (TFTA),triaminoguanidineazide, triaminoguanidine, and the double salt oftriaminoguanidineazide and hydrazine azide are loaded into the binder.Burning rate modifiers can also be incorporated in the binder.

When fluorine and interhalogen type oxidizers are employed, it was foundthat, with the exception of carbon, all of the components of the grainare either oxidized or liberated in gaseous form. The carbon, however,is liberated in the form of finely divided elemental carbon (i.e., soot)which not only produces an objectionable black smoky flame, but, also,reduces the performance from that which would have been obtained if thecarbon were oxidized.

Since oxygen is needed to oxidize the carbon, an approach to the problemhas been to modify the fluid oxidizer by including sufficient oxygen toreact with the carbon. Such fluid oxidizer systems include OF F LOX, andvarious interhalogen-oxygen compounds such as, for example, CIO F. Thesesystems, however, have not proven satisfactory, particularly wherestorability of the system is needed.

Another approach to the problem is to include an oxygen-containingcompound in the fuel grain. However, I have found that when theoxygen-containing compound is selected from, for example, the highperformance perchlorate type oxidizers now generally used in solid andhybrid propellants, such as ammonium perchlorate, it is not possible toincorporate sufficient oxidizer to react with all the carbon withoutcausing the fuel grain to sustain combustion after oxidizer flow hasbeen stopped. If a grain sustains, it is not suitable for use in anon-off system.

SUMMARY OF THE INVENTION According to this invention, it has been foundthat a hybrid propellant grain for use with halogen oxidizers can beloaded with an amount of oxygen-containing oxidizer stoichiometric withthe carbon content of the grain, without producing a sustaining grain ifthe oxidizer is selected from inorganic nitrates such as the alkali andalkaline earth nitrates and ammonium nitrate.

It is, accordingly, an object of this invention to provide a hybridpropulsion system employing a halogen or interhalogen oxidizer thatproduces a carbon-free exhaust.

It is another object of this invention to provide a hybrid propulsionsystem employing a halogen or interhalogen oxidizer that is capable ofstop-start operation.

It is another object of this invention to provide a fuelrich grain for ahybrid propulsion system that is nonsustaining.

It is another object of this invention to provide a nonsustaining hybridfuel grain containing amounts of oxidizer stoichiometric with the carboncontent of the grain.

These and other objects of this invention will be readily apparent fromthe following description of the invention.

DESCRIPTION OF THE INVENTION This invention can be employed with anypolymeric binder system such as those set out above, however, for easeof understanding, one binder system will be referred to throughout. Thebinder employed in the following examples consists of hydroxy-terminatedpolybutadiene cured with toluene diisocyanate which produces a tough,rubbery matrix incorporating the various additives.

In the combustion process, the carbon present is oxidized to CO and thefollowing equivalent weights are of use in determining the amounts ofthe various materials needed to achieve stoichiometry.

25.0g binder 1.85 equivalents of C 144.0g TFTA 4.00 equivalents of C80.5g NH NO 3.00 equivalents of O l 17.0g NI-I CIO 4.00 equivalents of 0Thus, in the simplest fuel system in which merely the fuel and oxidizerare present, it is seen that for 25g of binder, approximately 50g of NHNO are required for stoichiometric oxidation of the carbon present,whereas approximately 54g of NH CIO would be required. On a percentagebasis, this yields grains comprising, respectively, about 33 percentbinder and 67 percent NI-I NO and 32 percent binder and 68 percent NI-ICIO It is important to note that in no instance was it found possible toproduce a nonsustaining grain using NI-I CIO in amounts greater than 40percent by weight and, in most cases, sustaining was noted in the rangebetween 22 percent to 30 percent NH,CIO

As was noted above, it is generally desirable to incorporate varioushigh energy additives into the grain to enhance the performance of thesystem. The amount of these additives is generally determined by thesolids loading capability of the binder used, which, with techniquespresently available, is in the range of 75 percent to 90 percent byweight depending on the binder and the castability modifiers used. Thus,it is readily apparent that the performance of the basic system of thisinvention can be enhanced materially since the solids loading capabilityof the binder is not realized with the 33 percent binder 67 percentoxidizer grain. In this connection, it is also apparent that an amountof oxidizer substantially greater than that required to bestoichiometric with the carbon content of the grain is undesirable. Theexcess oxidizer takes the place of fuel material which could otherwisebe utilized and decreases the performance that could otherwise beobtained. The following examples are illustrative of various highperformance systems according to this invention. Grains having theformulations of Table l were fired with ClF and were found to benonsustaining upon shut-off of oxidizer flow. The percentages are byweight. The Nl-l NO used was Monsanto Dense E-2 fertilizer grade,average practice size about 2000p.

TABLE I No. Binder Nl-LNO; TFTA Boron Al Additional experiments withgrains containing boron or boron compounds have indicated that heatedboron appears to undergo an exothermic surface reaction with Nl-l NOwhich causes the grain to sustain if it is fired for a sufficient periodof time to heat the boron to the temperature required to initiate thereaction. Accordingly, it is preferable to limit the total boron contentof the grain to a maximum of 5 percent in those cases in which prolongedfiring periods are contemplated.

This invention has been described with respect to several embodimentsthereof. These embodiments are illustrative and not limiting of thisinvention. Various modifications will be apparent to workers skilled andcan be made without departing from the scope of this invention which islimited only by the following claims wherein:

I claim:

1. In a method of hybrid propulsion wherein a fuelrich grain is burnedwith a fluid oxidizer selected from the group consisting of fluorine andinterhalogen compounds to produce hot combustion products which areexhausted through a thrust producing nozzle and the flow rate of thefluid oxidizer is terminated at least once prior to the totalconsumption of said fuel-rich grain whereby intermittent propulsion maybe obtained; the improvement wherein said fuel-rich grain consists of acured carbonaceous polymeric binder having solid additives dispersedtherethrough, said solid additives comprising a material selected fromthe group consisting of ammonium nitrate, alkali metal nitrates, andalkaline earth metal nitrates, said material being present in an amountsuch that the oxygen content of the material is substantiallystoichiometric with the carbon cont nt of the r in whereb combustionroducts substantially free o? elemental carbon are btalned and saidgrain will extinguish upon termination of said fluid oxidizer flow.

2. The method of claim 1 wherein said material is ammonium nitrate.

3. The method of claim 1 wherein said additives further comprise solidhigh energy fuel materials selected from the group consisting of metals,metal hydrides, boron, boranes, organic borane derivatives, organichydrazine derivatives and heterocyclic nitrogen compounds.

4. The method of claim 3 wherein the boron content of said grain is nogreater than 5 percent by weight.

5. The method of claim 1 wherein said grain comprises at least about 10percent binder.

6. The method of claim 1 wherein said grain comprises from 20 33 percentby weight of hydroxy-terminated polybutadiene cured with toluenediisocyanate.

2. The method of claim 1 wherein said material is ammonium nitrate. 3.The method of claim 1 wherein said additives further comprise solid highenergy fuel materials selected from the group consisting of metals,metal hydrides, boron, boranes, organic borane derivatives, organichydrazine derivatives and heterocyclic nitrogen compounds.
 4. The methodof claim 3 wherein the boron content of said grain is no greater than 5percent by weight.
 5. The method of claim 1 wherein said grain comprisesat least about 10 percent binder.
 6. The method of claim 1 wherein saidgrain comprises from 20 -33 percent by weight of hydroxy-terminatedpolybutadiene cured with toluene diisocyanate.